In general, motors using permanent magnets are divided into surface permanent magnet (SPM) motors, in which magnets are attached to the surface of a rotor, and interior permanent magnet (IPM) motors, in which magnets are embedded inside a rotor. While SPM motors have a complicated structure in order to prevent magnets from flying apart during high-speed rotation, IPM motors do not have such a problem and thus are becoming the major type of motor used.
In IMP motors, the outer portion of the rotor that is adjacent to the stator is more influenced by an armature reaction, and permanent magnets embedded in the outer portion of the rotor are more vulnerable to irreversible demagnetization. In order to prevent the irreversible demagnetization, which occurs in the outer portion of the rotor, a portion of the permanent magnets, which are embedded in the outer portion of the rotor, may be removed. However, this leads to the problem of a decrease in power due to a reduction in the magnetic flux of the permanent magnets.